MEMORIES OF THE AERODYNAMIC DESIGN EFFORTS OF THE X-1 AT BELL AIRCRAFT

William H Pell, PhD

The X-1 was not meant to be a military plane, but from the outset was designed as an experimental plane for NACA. It was designed to attain supersonic speeds, and was to be heavily instrumented by NACA with the objective of obtaining data on high speed subsonic, transonic, and supersonic flight, and aerodynamic phenomena associated with it. There were, for instance, 400 pressure orifices on the wing, fuselage, and control surfaces. Propulsion was to be by rocket, and the plane would not take off from the ground, but would be dropped from a B-29. Mark Morkovin and I worked on the aerodynamic analysis of the plane's design (with help from others), a really exciting task, since we had to use every scrap of experimental evidence available, supplement this by theory, and then guess a little in predicting the behavior of the coefficients of lift, drag, and moment with changing Mach number. The interval 0.85<M<1.15 was virgin territory both for experiment and theory, which made things a bit scary. We were most gratified when flight tests with the plane later verified the essential correctness of our work.

The plan for dropping the X-1 from a B-29 was most unorthodox, of course, and was the source of much concern to us. The X-1 was to be suspended (partially) in the cutaway bomb bay of the B-29, and there was anxiety regarding the motion of the X-1 in the instant after its release. Would it drop cleanly away? Or would it foul the B-29? It was clear that the airflow about the X-1 suspended partially in the bomb bay but with the lower part of its fuselage and a portion of its wing projecting into the airstream, would be most complex, and not subject to even approximate theoretical analysis. I was part of a project to get some idea about this air flow by attaching small thread-like tufts at various points of the skin surface of the X-1, and observing their motion when the B-29 was in flight with the X-1 in position to be dropped. (This tuft technique is well-known in experimental fluid mechanics.) At the same time the pressure distribution over the surface of the X-1 was measured using the myriad of orifices with which it was equipped. I might add that the belly of the X-1 cleared the ground by about six inches when the B-29 was on the ground at rest, and that this, too, gave rise to some anxiety - at least to some of us!

The test flight was made in, I believe, February or March 1946. I was to observe the tufts visually through a glass window in the rear of the cutaway section in which the X-1 was suspended. The takeoff was without incident, and we spent about three hours flying to and fro above Lake Erie. There seemed to be no heat in the rear compartment, and I spent most of the time crouched miserably in my flying suit and Mae West, peering through the glass window and taking notes on what I saw. We were more jittery about the landing than the takeoff, but all went well.

No great illumination resulted from this exercise, as far as I recall, aside from reassurance regarding the B-29's ability to fly handily with its unwieldy load. It was decided, however, that the X-1 should not merely be dropped, but that it should be impulsively launched downward by a spring. I carried out an analysis of the subsequent motion, and we were happy to find when the actual launch occurred, that the initial instants of the X-1's motion were very close to our predictions.

The X-1 went on to fame of course. On October 14, 1947 it became the first manned airplane to exceed the speed of sound in level flight. In 1954 the X-1A attained a speed of 1600 mph , and in 1956 its descendant, the X-2, exceeded a Mach number of 3.0 (about 2060 mph).